Membrane made from a multi-block polymer comprising an imide or amide-acid prepolymer chain extended with a compatible second prepolymer and its use in separations

ABSTRACT

The present invention is directed to a multi-block polymeric material comprising an imide prepolymer chain extended with a second, different, compatible prepolymer selected from the group of prepolymers comprising (a) an (A) dianhydride or its corresponding tetraacid or diacid - diester combined with a monomer selected from (B) epoxy, diisocyanate, polyester and diamine in an A/B mole ratio ranging from about 2.0 to 1.05, preferably about 2.0 to 1.1 and (b) an (A) diamine combined with a monomer selected from (B) epoxy, diisocyanate and dianhydride or its corresponding tetraacid or diacid - diester in an A/B mole ratio ranging from about 2.0 to 1.05, preferably about 2.0 to 1.1, and mixtures thereof. The present invention is also directed to membranes of the above recited multi-block, polymeric material, especially membranes compising thin, dense films of said multi-block polymeric material deposited on a microporous support layer producing a thin film composite membrane. The membranes of the multi-block polymeric material, especially the thin film composite membranes, are useful for separating aromatic hydrocarbons from mixtures of same with non-aromatic hydrocarbons under perstraction or pervaporation conditions.

DESCRIPTION OF THE INVENTION

The present invention is directed to a multi-block polymeric materialcomprising an imide or amide-acid prepolymer chain extended in an about1 to 1 mole ratio with a second, different, compatible prepolymerselected from the group of prepolymers comprising (a) an (A) dianhydrideits corresponding tetraacid or its diacid-diester combined with amonomer selected from (B) epoxy, diisocyanate, polyester, and diamine inan A/B mole ratio ranging from about 2.0 to 1.05, preferably about 2.0to 1.1 and (b) an (A) diamine combined with a monomer selected from (B)epoxy, diisocyanate and dianhydride, its corresponding tetra-acid, orits diacid-diester in an A/B mole ratio ranging from about 2.0 to 1.05,preferably about 2.0 to 1.1, and mixtures thereof.

The present invention is also directed to membranes of the above recitedmulti-block polymeric material, especially membranes comprising thin,dense, nonporous films of said multi-block polymeric material depositedon a microporous support layer producing a thin film composite membrane.

The membranes of the multi-block polymeric material, especially the thinfilm composite membranes, are useful for separating aromatichydrocarbons including heteroatom containing aromatics from mixtures ofsame with non-aromatic hydrocarbons under perstraction or pervaporationconditions.

BACKGROUND OF THE INVENTION

Polyurea/urethane membranes and their use for the separation ofaromatics from non-aromatics are the subject of U.S. Pat. No. 4,914,064.In that case the polyurea/urethane membrane is made from apolyurea/urethane polymer characterized by possessing a urea index of atleast about 20% but less than 100%, an aromatic carbon content of atleast about 15 mole percent, a functional group density of at leastabout 10 per 1000 grams of polymer, and a C=O/NH ratio of less thanabout 8.0. The polyurethane/urea multi-block copolymer is produced byreacting dihydroxy or polyhydroxy compounds, such as polyethers orpolyesters having molecular weights in the range of about 500 to 5,000with aliphatic, alkylaromatic or aromatic diisocyanates to produce aprepolymer which is then chain extended using diamines, polyamines oramino alcohols. The membranes are used to separate aromatics fromnon-aromatics under perstraction or pervaporation conditions.

The use of polyurethane imide membranes for aromatics from non-aromaticsseparations is disclosed in U.S. Pat. No. 4,929,358. Thepolyurethane-imide membrane is made from a polyurethane-imide copolymerproduced by end capping a polyol such as a dihydroxy or polyhydroxycompound (e.g. polyether or polyester) with a di or polyisocyanate toproduce a prepolymer which is then chain extended by reaction of saidprepolymer with a di or polyanhydride or with a di or polycarboxylicacid to produce a polyurethane amic acid which is then chemically orthermally cyclized to the imide. The aromatic/non-aromatic separationusing said membrane is preferably conducted under perstraction orpervaporation conditions.

A polyester imide copolymer membrane and its use for the separation ofaromatics from non-aromatics is the subject of U.S. Pat. No. 4,946,594.In that case the polyester imide is prepared by reacting polyester witha dianhydride to produce a prepolymer which is then chain extended witha diisocyanate to produce the polyester imide.

The use of membranes to separate aromatics from saturates has long beenpursued by the scientific and industrial community and is the subject ofnumerous patents.

U.S. Pat. No. 3,370,102 describes a general process for separating afeed into a permeate stream and a retentate stream and utilizes a sweepliquid to remove the permeate from the face of the membrane to therebymaintain the concentration gradient driving force. The process can beused to separate a wide variety of mixtures including various petroleumfractions, naphthas, oils, hydrocarbon mixtures. Expressly recited isthe separation of aromatics from kerosene.

U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type,i.e. aromatic, unsaturated, saturated, by permeating a portion of themixture through a non-porous cellulose ether membrane and removingpermeate from the permeate side of the membrane using a sweep gas orliquid. Feeds include hydrocarbon mixtures, naphtha (including virginnaphtha, naphtha from thermal or catalytic cracking, etc.).

U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbonse.g. aromatic and/or olefins from gasoline boiling range mixtures, bythe selective permeation of the aromatic through certain cellulose esternon-porous membranes. The permeated hydrocarbons are continuouslyremoved from the permeate zone using a sweep gas or liquid.

U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes toselectively separate aromatics from saturates via pervaporation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises a multi-block polymeric materialcomprising a first prepolymer made by combining an (A) diisocyanate with(B) dianhydride or its corresponding tetraacid or its diacid-diester inan A/B mole ratio ranging from about 2.0 to 1.05, preferably about 2.0to 1.1 to produce an imide or amide-acid which is subsequently chainextended in an about 1 to 1 mole ratio with a second, different andcompatible prepolymer selected from the group of prepolymers comprising(a) an (A) dianhydride, its corresponding tetraacid or itsdiacid-diester combined with a monomer selected from (B) epoxy,diisocyanate, polyester and diamine in an A/B mole ratio ranging fromabout 2.0 to 1.05, preferably about 2.0 to 1.1 and (b) an (A) diaminecombined with a monomer selected from (B) epoxy, diisocyanate anddianhydride, its corresponding tetraacid or its diacid-diester in an A/Bmole ratio ranging from about 2.0 to 1.05, preferably about 2.0 to 1.1,and mixtures thereof.

The multi-block prepolymeric material can be formed into a thickmembrane layer or deposited as a thin film on a microporous supportresulting in the production of a thin film composite membrane.

The membranes can be used to separate aromatic hydrocarbons frommixtures of same with non-aromatic hydrocarbons under perstraction orpervaporation conditions, as used hereinafter in this text and theappended claims, the term "aromatic hydrocarbons" is understood asmeaning to include single and multi-ring side chain bearing andunsubstituted aromatics containing only carbon and hydrogen, single andmulti-ring side chain bearing and unsubstituted heterocyclic aromaticssuch as thiophene, pyridine, quinoline, benzothiophenes, benzofuran,etc., and single and multi-ring aromatic and heterocyclic aromaticsbearing heteroatom substituted side chains.

In preparing the multi-block polymeric material one begins by preparinga first prepolymer by combining a diisocyanate (A) with a dianhydride(B), its corresponding tetraacid or its diacid-diester in a mole ratio(A/B) ranging from 2.0 to 1.05, preferably 2.0 to 1.1. The reaction ofthe dianhydride with a diisocyanate yields an imide while the reactionof a tetraacid or diacid-diester with a diisocyanate yields anamide-acid.

Aliphatic and cycloaliphatic di and polyisocyanate can be used as can bemixtures of aliphatic, cycloaliphatic, aralkyl and aromaticpolyisocyanates.

The diisocyanates are preferably aromatic diisocyanates having thegeneral structure: ##STR1## wherein R' and R'' are the same or differentand are selected from the group consisting of H, C₁ -C₅ and C₆ H₅ andmixtures thereof and n ranges from 0 to 4.

Aliphatic diisocyanates which may be utilized are exemplified byhexamethylene diisocyanate (HDI),1,6-diisocyanato-2,2,4,4-tetramethylhexane (TMDI), 1,4-cyclohexanyldiisocyanate (CHDI), isophorone diisocyanate (IPDI), while usefulalkylaromatic diisocyanates are exemplified by 2,4-toluene diisocyanate(TDI) and bitolylene diidocyanate (TODI). Aromatic diisocyanates areexemplified by 4,4'-diisocyanate diphenylmethane (MDI), methylenedichlorophenyl diisocyanate (dichloro MDI), methylene dicyclohexyldiisocyanate (H₁₂ -MDI), methylene bis [dichlorophenyl isocyanate](tetrachloro MDI), and methylene bis [dichlorocyclohexyl isocyanate](tetrachloro - H₁₂ MDI). Polyisocyanates are exemplified by polymericMDI (PMDI) and carbodiimide modified MDI and isocyanurate isocyanates.

Dianhydrides or tetracarboxylic acids or diacid-diesters which produceamide acid groups are also used in producing the prepolymer.

Any aromatic, aliphatic, cycloaliphatic or araliphatic dianhydride canbe used. Examples of di anhydrides include by way of example and notlimitation: pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride,4,4'-(hexafluoroisopropylidene)-bis-(phthalic anhydride),4,4'-oxydiphthalic anhydride, diphenylsulfone-3,3'4,4'-tetracarboxylicdianhydride, and 3,3',4,4'-biphenyltetracarboxylic dianhydride.

The diisocyanate and dianhydride, its corresponding tetra-acid ordiacid-diester are combined in an about 2 to 1.05, preferably 2 to 1.1mole ratio. The reaction between these two reactants is run tocompletion. Completion of the reaction can be determined by anytechnique known to those skilled in the art. For example, thedisappearance of isocyanate groups can be monitored by infraredspectroscopy. The reaction can be run neat but generally is run in thepresence of an added polar, aprotic solvent such as DMF, NMP, DMAC,DMSO, etc.

The second, compatible prepolymer which is added to the imide oramide-acid in an about 1 to 1 mole ratio is selected from the group ofprepolymers comprising (a) an (A) dianhydride, its correspondingtetraacid or diacid-diester combined with a monomer selected from (B)epoxy, diisocyanate, polyester and diamine in an A/B mole ratio rangingfrom about 2.0 to 1.05, preferably about 2.0 to 1.1, and (b) an (A)diamine combined with a monomer selected from (B) epoxy, diisocyanate,dianhydride its corresponding tetraacid or its diacid-diester in an A/Bmole ratio ranging from about 2.0 to 1.05, preferably about 2.0 to 1.1,and mixtures thereof.

The diisocyanates and dianhydrides, their corresponding tetraacids ortheir diacid-diesters used in preparing these second prepolymers areselected from the same groups of diisocyanates and dianhydrides, theircorresponding tetraacids or their diacid-diesters described previouslyfor the production of the first prepolymer imide.

Dicarboxylic acid/diester and tetracarboxylic acid derivatives ofdianhydrides when used must first be converted to species that willreact with diamines or polyesters. This can be done by conversion of thedicarboxylic acid/diester or tetracarboxylic moieties to (1) acidchlorides via derivatization with e.g. thionyl chloride or to (2)diimidazoles via reaction with e.g. carbonyl diimidazole. Subsequentreaction of the derivatized prepolymer with (1) diamines results information of an amide acid which must then be thermally or chemicallycyclized to form the imide, or (2) polyesters results in the formationof an ester which requires no further curing.

The epoxy used to produce the second prepolymer has the general formula:##STR2## R may be any saturated, unsaturated, or aromatic group, halogensubstituted saturated, unsaturated or aromatic group as well as groupscontaining oxygen in the form of ether linkages, and mixtures thereof.Representative of useful epoxy compounds are the following: ##STR3##identified as DER332 from Dow Chemical and ##STR4## identified as DER542from Dow Chemical

Polyesters having molecular weights in the range of about 500 to 5000can be used in preparing the second compatible prepolymer.

The polyester components are prepared from aliphatic or aromaticdicarboxylic acids and aliphatic or aromatic dialcohols. Aliphaticdicarboxylic acids refer to those materials having the general formulaHOOCRCOOH where R contains 2 to 10 carbons (and may be either a straightor branched chain configuration). Aromatic dicarboxylic acids refer tothose materials having the general structure HOOCRCOOH where R is:##STR5## wherein R', R'', and R''' may be the same or different and areselected from the group consisting of H and C₁ -C₅ carbons or C₆ H₅ andcombinations thereof, and n is 0 to 4. It is to be understood that inthe above formula each R' or R'' may itself represent a mixture of H, C₁-C₅ or C₆ H₅.

Dialcohols have the general structure HOROH where R may be ##STR6##where n is 1 to 10, preferably 4 to 6, and R' is H, C₁ to C₅ or C₆ H₅ or##STR7## where R', R'', and R''' and n are defined in the same manner asfor the aromatic dicarboxylic acids. An example of a useful dialcohol isbisphenol A.

Diamines which can be used have the general formula H₂ NRNH₂ where Rincludes aliphatic and aromatic moieties, such as ##STR8## where n is 1to 10 and R' may be the same or different and are selected from thegroup consisting of H, C₁ -C₅ carbons and C₆ H₅ and mixtures thereof.

Also included are diamines of the formula: ##STR9## where R', R'' andR''' are the same or different and are selected from the groupconsisting of H or C₁ or a C₁ to C₅ or C₆ H₅ and mixtures thereof and nranges from 0 to 4.

Useful polyamines are exemplified by polyethyleneimines and 2,2',2''triaminotriethylamine. Useful amino alcohols are exemplified by6-aminohexanol, 4-aminophenol, 4-amino-4,-hydroxydiphenylmethane.

In each instance the appropriate monomeric materials in theaforementioned mole ratios are combined to produce the desired secondprepolymer. Depending on the physical nature of the second prepolymerthe reagents are combined and either reacted to completion or to a pointshort of completion. The reaction is run to completion when such secondprepolymer exists in liquid or solution in solvent form. If however thesecond prepolymer when run to completion is in the form of a solid orinsoluble gel then reaction to completion is unacceptable. In suchinstances the reagents are reacted until just before the viscosity ofthe reaction mixture becomes too difficult to manage. The secondprepolymer is then, combined with the first prepolymer.

The first and second prepolymers can be reacted neat, that is, in theabsence of added solvent, if their individual natures favor such absenceof solvent, or the reaction can be run in the presence of a solventappropriate for the polymerization conditions employed. In general thereaction will be run in a solvent which may be selected from any of thepolar, aprotic solvents such as tetrahydrofuran (THF), DMAC, DMSO, DMF,as well as NMP and cellosolve acetate.

Certain combinations of first prepolymer and second prepolymer may bereacted to completion while other combinations must be used to cast amembrane before the reaction goes to completion, i.e. while the reactantsolution is still of a manageable viscosity and before formation of agel. In those instances, which can be determined by the practitionerusing the information before him in this specification without theexpenditure of any inventive effort, the solution is spread or poured onthe appropriate support, the copolymer layer on support inserted into anoven to drive off the casting solution solvent then heated to atemperature for a time sufficient to drive the polymerization reactionto completion and cure the membrane.

The multi-block polymer in solvent or dissolved in added solvent is usedas a casting solution. Polymer concentration in solvent ranges from 10to 70 wt% preferably 15%-50 wt% for casting dense films. When castingintegral thin film composite membranes, e.g. thin layers of polymerpreferably about 0.1 to 5.0 microns thick on micro-porous supportbackings such as ceramic, sintered glass or metal or polymeric materialsuch as nylon, porous polypropylene, porous Teflon®, or porous polyurea,preferably porous Teflon®, the polymer concentration in solution is onthe order of about 50% or less.

The casting solution is poured or spread on an appropriate supportmedium, such as a metal or glass plate or, if desired, a woven fiberbacking, such as woven fiber glass, nylon, polyester, etc. can be usedif solvent removal during the casting sequence employs a vacuum, butpreferably, non-woven backings such as thin films of porouspolypropylene, porous urea or porous Teflon® are employed. In general,however, backing materials used are those which are not attacked by thesolvent(s) used to produce the copolymer casting solution and which cansurvive in the environment (chemical and thermal) to which the membranewill be exposed.

The membrane may be cast in any thickness, membranes ranging inthickness of from about 0.1 to about 50 microns being preferred, thethin, dense layer in the composite membrane being preferably about 0.1to 5.0 microns thick.

A very thin layer of the multi-block polymer can be deposited onto ahighly permeable, non-selective layer producing a composite membranecomprising a thin dense (nonporous) layer of multi-block polymermembrane about 0.1 to 5 microns thick on a permeable, non-selective,thick backing. The thick underlayer (about 20 to 100 microns thick)serves as a support layer permitting one to produce thin, dense,selective layers of multiblock polymer membranes which would otherwisebe mechanically unmanageable due to their thinness. In many instances,due to the chemical similarity between the support layer and theselective layer, the two layers interact through hydrogen bonding toproduce a very strong bond, in addition to physical adhesion. Forgeneral low temperature applications the porous, non-selective backingneed not be capable of operation at high temperatures. In such service,such as the perstractive separation of aromatics from non-aromaticsbacking such as polyurethane or polyurea/urethane would be sufficient.For higher temperature applications, of course, the backing materialmust itself be capable of remaining intact at the high temperature. Forsuch applications backings such as polyester/imide, Teflon® or evenceramics, sintered glass or metal supports should be used.

If one were to use this technique to produce sheet material, the thick,permeable underlayer can be deposited on a suitable casting backingmaterial such as porous fiber glass, polyethylene, polypropylene, nylon,teflon, etc. after which the thin, dense selective layer would bedeposited onto the under layer. The casting backing material can then beremoved leaving the composite sheet membrane.

In producing hollow fibers or tubes using this composite membranetechnique, first a tube or fiber of permeable material such aspolyurethane is produced after which a thin dense layer of themulti-block polymer material is deposited on either the outer or innersurface of the tube or fiber support.

A permeable polyurethane layer can be prepared from polyether glycolssuch as polypropylene glycol or polybutylene glycol plus aliphaticand/or aromatic diisocyanates (preferably aliphatic diisocyanates) usingpolyols (diols or triols) preferably aliphatic diols as chain extenders.Polyurethane membrane materials which satisfy the above requirement ofpermeability are the polyurethane membranes described in U.S. Pat. No.4,115,465.

The membranes are useful for the separation of aromatics fromnon-aromatics in petroleum and chemical streams, and have been found tobe particularly useful for the separation of larger, substitutedaromatics from non-aromatics as are encountered in heavy cat naphthastreams. Other streams which are also suitable feed streams foraromatics from saturates separation are intermediate cat naphtha streams(200°-320° F.), light aromatics content streams boiling in the C₅ -300°F. range, light catalytic cycle oil boiling in the 400°-650° F. range,reformate streams as well as streams in chemical plants which containrecoverable quantities of benzene, toluene, xylene (BTX) or otheraromatics in combination with saturates. The separation techniques whichmay successfully employ the membranes of the present invention includeperstraction and pervaporation.

Perstraction involves the selective dissolution of particular componentscontained in a mixture into the membrane, the diffusion of thosecomponents through the membrane and the removal of the diffusedcomponents from the downstream side of the membrane by use of a liquidsweep stream. In the perstractive separation of aromatics from saturatesin petroleum or chemical streams (particularly heavy cat naphthastreams) the aromatic molecules present in the feedstream dissolve intothe membrane film due to similarities between the membrane solubilityparameter and those of the aromatic species in the feed. The aromaticsthen permeate (diffuse) through the membrane and are swept away by asweep liquid which is low in aromatics content. This keeps theconcentration of aromatics at the permeate side of the membrane film lowand maintains the concentration gradient which is responsible for thepermeation of the aromatics through the membrane.

The sweep liquid is low in aromatics content so as not to itselfdecrease the concentration gradient. The sweep liquid is preferably asaturated hydrocarbon liquid with a boiling point much lower or muchhigher than that of the permeated aromatics. This is to facilitateseparation, as by simple distillation. Suitable sweep liquids, therefor,would include, for example, C₃ to C₆ saturated hydrocarbons and lubebasestocks (C₁₅ -C₂₀).

The perstraction process is run at any convenient temperature,preferably as low as possible.

The choice of pressure is not critical because the perstraction processis not dependent on pressure, but on the ability of the aromaticcomponents in the feed to dissolve into and migrate through the membraneunder a concentration driving force. Consequently, any convenientpressure may be employed, the lower the better to avoid undesirablecompaction, if the membrane is supported on a porous backing, or ruptureof the membrane, if it is not.

If C₃ or C₄ sweep liquids are used at 25° C. or above in liquid state,the pressure must be increased to keep them in the liquid phase.

Pervaporation, by comparison, is run at generally higher temperaturesthan perstraction with the feed being in either the liquid or vaporstate and relies on vacuum or a sweep gas on the permeate side toevaporate or otherwise remove the permeate from the surface of themembrane and maintain the concentration gradient driving force whichdrives the separation process. As in perstraction, the aromaticmolecules present in the feed dissolve into the membrane film, migratethrough said film and reemerge on the permeate side under the influenceof a concentration gradient. Pervaporation separation of aromatics fromsaturates can be performed at a temperature of about 25° C. for theseparation of benzene from hexane but for separation of heavieraromatic/saturate mixtures, such as heavy cat naphtha, highertemperatures of at least 80° C. and higher, preferably at least 100° C.and higher, more preferably 120° C. and higher (up to about 170 to 200°C. and higher) can be used, the maximum upper limit being thattemperature at which the membrane is physically damaged. Vacuum on theorder of 1-50 mm Hg is pulled on the permeate side. The vacuum streamcontaining the permeate is cooled to condense out the highly aromaticpermeate. Condensation temperature should be below the dew point of thepermeate at a given vacuum level.

The membrane itself may be in any convenient form utilizing anyconvenient module design. Thus, sheets of membrane material may be usedin spiral wound or plate and frame permeation cell modules. Tubes andhollow fibers of membranes may be used in bundled configurations witheither the feed or the sweep liquid (or vacuum) in the internal space ofthe tube or fiber, the other material obviously being on the other side.

Most conveniently, the membrane is used in a hollow fiber configurationwith the feed introduced on the exterior side of the fiber, the sweepliquid flowing on the inside of the hollow fiber to sweep away thepermeated highly aromatic species, thereby maintaining the desiredconcentration gradient. The sweep liquid, along with aromatics containedtherein, is passed to separation means, typically distillation means,however, if a sweep liquid of low enough molecular weight is used, suchas liquefied propane or butane, the sweep liquid can be permitted tosimply evaporate, the liquid aromatics being recovered and the gaseouspropane or butane (for example) being recovered and reliquefied byapplication of pressure or lowering the temperature.

The present invention will be better understood by reference to thefollowing Examples which are offered by way of illustration and notlimitation.

EXAMPLE 1

An imide prepolymer was made by adding 1.74 grams (0.01 mole) toluenediisocyanate (TDI) to 1.32 grams (0.005 mole) of5-[2,5-dioxotetrahydro-3furanyl]-3-cyclohexene-1,2-dicarboxylicanhydride (B-4400) dissolved in 7.114 grams NMP. It was heated to 100°C. and held for 2 hours until all CO₂ evolution had ceased.

EXAMPLE 2

An epoxy prepolymer was made by adding 20.92 grams (0.051 mole) of2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) to 29.56 grams NMP tomake a solution of diamine. To that was added 8.67 grams (0.0255 mole)diglycidyl ether of Bisphenol A (DER-332) and the mixture was stirredand heated for 5 hours at 51° C.

EXAMPLE 3

Eleven point six (11.6) grams (0.005 moles) of the prepolymer fromExample 2 was mixed with all of the prepolymer from Example 1. It wasallowed to react for 1 hour at room temperature and then cast onto amicroporous Teflon® support. Solvent was evaporated in a vacuum oven at100° C. and the membrane was post-heat treated at 150° C. for 1 hour.Average thickness of the membrane was 68 microns. It was tested for thepervaporative separation of aromatics from saturates in a model feedmixture containing 10 wt% toluene, 40 wt.% p-xylene, 20 wt.% isooctaneand 30 wt.% n-octane. Helium was used as the permeate sweep gas at 50cc/min. Performance was measured at temperatures ranging from 140°-180°C. and the results are shown in Table I below demonstrating that thesemembranes are effective in the separation of aromatics fromnon-aromatics. ##EQU1##

                  TABLE I                                                         ______________________________________                                        Temper-                                                                       ature  Selectivity            Permeability                                    (°C.)                                                                         Toluene/n-Octane                                                                           p-Xylene/n-Octane                                                                           (kg-μ/m.sup.2 /d)                        ______________________________________                                        140    8.83         4.94            5.74                                      150    4.79         2.84           12.52                                      160    7.21         4.13           36.17                                      170    10.40        6.33          100.2                                       180    10.05        6.85          206.0                                       ______________________________________                                    

EXAMPLE 4

An imide prepolymer like that in Example 1 was prepared. One pointseven-four grams (0.01 mole) toluene diisocyanate was mixed with 6.00grams NMP. Then 1.32 grams (0.005 mole) B-4400 was dissolved into 6.24grams NMP. These two were added together and heated for 2 hours at 100°C. until all CO₂ evolution had ceased.

EXAMPLE 5

An epoxy prepolymer was next prepared. To 9.6 grams NMP was added 6.2grams (0.02 mole) of the diethyl ester of pyromellitic dianhydride(DEEPMDA). To this was added 3.4 grams (0.01 mole) DER-332 epoxy whichwas mixed to dissolve well. The reaction mixture was heated withstirring at 100° C. for 4 hours. Epoxy analysis showed no epoxy endgroups present in the product.

EXAMPLE 6

One half (7.65 grams) of the prepolymer solution from Example 5 wasadded to all of the prepolymer from Example 4 and the mixture was heatedat 70° C. for 1 hour. The resulting solution was cast onto microporousTeflon®, dried in vacuum oven overnight at 100° C. and full vacuum andpost cured at 160° C. for 1 hour in air.

A piece of the resulting film (approximately 54 microns thick) wastested for the pervaporative separation of aromatics from non-aromaticsusing the feed described in Example 3 and the results, shown in Table IIbelow, indicate the desired separation.

                  TABLE II                                                        ______________________________________                                        Temper-                                                                       ature  Selectivity            Permeability                                    (°C.)                                                                         Toluene/n-Octane                                                                           p-Xylene/n-Octane                                                                           (kg-μ/m.sup.2 /d)                        ______________________________________                                        140    2.04         1.46          11.7                                        150    2.09         1.38          14.4                                        160    2.36         1.54          17.3                                        170    2.94         1.83          19.2                                        180    2.54         1.60          21.0                                        ______________________________________                                    

EXAMPLE 7

A polyamic acid prepolymer was prepared by dissolving 2.05 grams (0.005mole) BAPP in 11.16 grams NMP. To this was 0.74 gram (0.0025 mole)biphenyl dianhydride and the mixture was allowed to react at 25° C.overnight.

EXAMPLE 8

Another batch of imide prepolymer from Example 4 was prepared and theresulting solution added to the prepolymer from Example 7. The mixturestarted gelling within 2 minutes of the time of addition so that it wasquickly cast onto microporous Teflon. It was dried under flowingnitrogen for 1 hour, then under full vacuum at 100° C. for 1 hour andpost cured in air at 150° C. for 1 hour. A piece of this film(approximately 24 microns thick) was tested for the pervaporativeseparation of aromatics from non-aromatics using the feed previouslydescribed and the results are shown in Table III below. As can be seen,this membrane is effective for the desired separation.

                  TABLE III                                                       ______________________________________                                        Temper-                                                                       ature  Selectivity            Permeability                                    (°C.)                                                                         Toluene/n-Octane                                                                           p-Xylene/n-Octane                                                                           (kg-μ/m.sup.2 /d)                        ______________________________________                                        140    2.08         1.65           3.9                                        150    2.75         1.86          10.8                                        160    3.10         2.04          14.2                                        170    3.71         2.41          14.9                                        180    4.09         2.72          19.5                                        ______________________________________                                    

What is claimed is:
 1. A method for separating aromatic hydrocarbonsfrom feed streams comprising mixtures of aromatic hydrocarbons andnon-aromatic hydrocarbons, said method comprising contacting the feedstream with one side of a membrane made from a multi block polymermaterial comprising a first prepolymer comprising an imide or amide-acidmade by reacting an (A) diisocyanate with (B) a dianhydride, itscorresponding tetraacid or diacid - diester in an A/B mole ratio rangingfrom about 2.0 to 1.05, chain extended in an about 1 to 1 mole ratiowith a second, different and compatible prepolymer selected from thegroup of prepolymers comprising: (a) an (A) dianhydride or itscorresponding tetraacid or diacid - diester combined with a monomerselected from (B) epoxy, diisocyanate, polyester and diamine in an A/Bmole ratio ranging from about 2.0 to 1.05, and (b) an (A) diaminecombined with a monomer selected from (B) epoxy, diisocyanate anddianhydride, its corresponding tetraacid or diacid diester in an A/Bmole ratio ranging from about 2.0 to 1.05, and mixtures thereof, saidseparation being conducted under pervaporation or perstractionconditions, whereby the aromatic hydrocarbon component of the feedstream selectively permeates through the membrane.
 2. The method ofclaim 1 wherein the membrane comprises a thin, dense film of saidmulti-block polymeric material deposited on a microporous support layerproducing a thin film composite membrane.
 3. The method of claim 2wherein the microporous support layer is nylon, porous polypropylene,porous Teflon®, porous polyurea or porous polyurethane.
 4. The method ofclaim 1 wherein the membrane layer ranges from about 0.1 to about 50microns in thickness.
 5. The method of claim 2 or 3 wherein the membranelayer ranges from about 0.1 to 5.0 microns in thickness.
 6. A membranemade of a multi-block polymer comprising a first prepolymer comprisingan imide or amide-acid made by reacting an (A) diisocyanate with (B) adianhydride or its corresponding tetraacid or diacid - diester in an A/Bmole ratio ranging from about 2.0 to 1.05, chain extended in an about 1to 1 mole ratio with a second, different, and compatible prepolymerselected from the group of prepolymers comprising (a) an (A) dianhydrideor its corresponding tetraacid or diacid - diester combined with amonomer selected from (B) epoxy, diisocyanate, polyester, and diamine inan A/B mole ratio ranging from about 2.0 to 1.05, and (b) an (A) diaminecombined with a monomer selected from (B) epoxy, diisocyanate, anddianhydride, its corresponding tetraacid or diacid - diester in an A/Bmole ratio ranging from about 2.0 to 1.05, and mixtures thereof.
 7. Themembrane of claim 6 comprising a thin, dense film of said multi-blockpolymeric material deposited on a microporous support layer producing athin film composite membrane.
 8. The composite membrane of claim 7wherein the micro porous support layer is nylon, porous polypropylene,porous Teflon®, porous polyurea or porous polyurethane.
 9. The membraneof claim 6 wherein the membrane layer ranges from about 0.1 to about 50microns in thickness.
 10. The membrane of claim 7 or 8 wherein themembrane layer ranges from about 0.1 to 5.0 microns in thickness.